There exists several different radio based navigation systems of which the GPS system is the most well known. GPS is an abbreviation of Global Positioning System and is a global covering and from satellite transmitted radio based navigation system. The GPS system provides high accuracy and has in principle two accuracy classes where full accuracy only is available for the military GPS. The accuracy is restricted for civilian use where techniques such as differential GPS is developed to improve the accuracy. Differential GPS provides improved location accuracy compared to ordinary GPS by using radio signals from a network of fixed ground base reference stations with a known position. The information transmitted from the ground base reference stations is the difference between the measured satellite distance (pseudorange) and the actual distance. The receiver of the signal from the ground based reference stations may correct their measured distances (pseudoranges) by the correction factor defined in the signal transmitted from the ground based reference stations.
It is also known that combining the radio based positioning system with a dead-reckoning or inertial based navigation system could improve the accuracy of the radio based systems at occasions where the radio based navigation systems received radio signal is expected to be unavailable or distorted.
It is also known that a map or a database with position information could be used to improve the accuracy of the position received by a GPS system. The invention published in patent document WO00/50917 discloses a navigation system for a vehicle for correcting the intermittent absence of the GPS signal and correction of the GPS errors with navigation information from other system. The specified other systems are dead-reckoning such as velocity of the vehicle and a map with position information. Correction of the accuracy of the GPS position is made when the vehicle is moving at a junction, such as a turning point or crossing, where the position in the map is identified and a correction is made to the, by the GPS receiver, received position.
Problem with the solution described in WO00/50917 for improved navigation accuracy is the difficulty to identify the map location that is used to correct the position received by the GPS receiver.
Accordingly, it is an object of the invention to provide a reliable and improved method to correct the received position.
Other problems solved by the invention are described in the detailed description,